Comparison of UV and visible Raman spectroscopy of bulk metal molybdate and metal vanadate catalysts.

The visible (532 and 442 nm) and UV (325 nm) Raman spectra of bulk mixed metal oxides (metal molybdates and metal vanadates) were compared on the same spectrometer, for the first time, to allow examination of how varying the excitation energy from visible to UV affects the resulting Raman spectra. The quality of the Raman spectra was found to be a strong function of the absorption properties of the bulk mixed oxide. For bulk mixed metal oxides that absorb weakly in the visible and UV regions, both the visible and UV Raman spectra were of high quality and exhibit identical vibrational bands, but with slightly different relative intensities. For bulk mixed metal oxides that absorb strongly in the UV and visible regions and/or strongly in the UV and weakly in the visible regions, the visible Raman spectra are much richer in structural information and of higher resolution than the corresponding UV Raman spectra. This is a consequence of the strong UV absorption that significantly reduces the sampling volume and number of scatterers giving rise to the Raman signal. The shallower escape depth of UV Raman, however, was not sufficient to detect vibrations from the surface metal oxide species that are present on the outermost surface layer of these crystalline mixed metal oxide phases as previously suggested. It was also demonstrated that there is no sample damage by the more energetic UV excitation when very low laser powers and fast detectors are employed, thus avoiding the need of complicated fluidized bed sample arrangements sometimes used for UV Raman investigations. The current comparative Raman investigation carefully documents, for the first time, the advantages and disadvantages of applying different excitation energies in collecting Raman spectra of bulk mixed metal oxide materials.